CN110911656B - Lithium secondary battery - Google Patents

Lithium secondary battery Download PDF

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CN110911656B
CN110911656B CN201811466269.3A CN201811466269A CN110911656B CN 110911656 B CN110911656 B CN 110911656B CN 201811466269 A CN201811466269 A CN 201811466269A CN 110911656 B CN110911656 B CN 110911656B
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silicon
fec
lithium secondary
secondary battery
electrolyte
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CN110911656A (en
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朴相镇
吴承旼
林成勋
李世英
孔信国
赵廷英
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Hyundai Motor Co
Kia Corp
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Kia Motors Corp
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    • H01M2220/00—Batteries for particular applications
    • H01M2220/20—Batteries in motive systems, e.g. vehicle, ship, plane
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    • H01M2300/0025—Organic electrolyte
    • H01M2300/0028—Organic electrolyte characterised by the solvent
    • H01M2300/0034—Fluorinated solvents
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
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    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

Disclosed is a lithium secondary battery capable of preventing a reduction in battery life. The lithium secondary battery includes a cathode, an anode comprising silicon, a separator between the cathode and the anode, and an electrolyte comprising fluoroethylene carbonate (FEC), wherein the weight ratio of silicon to FEC is about 0.4 to about 0.8.

Description

Lithium secondary battery
Technical Field
The present invention relates to a lithium secondary battery having improved battery life.
Background
In the related art, a lithium secondary battery containing an electroactive material has a higher operating voltage and a high energy density than a lead battery or a nickel/cadmium battery. Accordingly, lithium secondary batteries have been used as energy storage devices for Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs).
In order to improve the travel distance of an electric vehicle, the battery is required to have a high energy density, and for this reason, the material for the battery is required to have an improved energy density. Currently, lithium secondary batteries using Ni, co, and Mn-based cathode materials and graphite-based anode materials have been developed, and alternative materials that can replace these materials have also been developed to improve the limitations of energy density. For example, silicon has been developed with a high energy density, having a capacity of 4000mAh/g, greater than the capacity of graphite (360 mAh/g).
Disclosure of Invention
In a preferred aspect, there is provided, among other things, a lithium ion battery without degrading the performance of the unit cell by effectively forming a passivation film on silicon.
Additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
In one aspect of the present invention, there is provided a lithium secondary battery including: a cathode; an anode comprising a silicon component; a separator disposed between the cathode and the anode; an electrolyte comprising a fluorocarbonate. Preferably, the electrolyte may suitably comprise fluoroethylene carbonate (FEC). Preferably, the weight ratio of silicon to FEC may be about 0.4 to about 0.8.
As used herein, the term "silicon component" includes materials containing silicon as a major component that constitute an amount greater than about 10wt%, 20wt%, 30wt%, 40wt%, 50wt%, 60wt%, 70wt%, 80wt%, 90wt%, 95wt%, or 99wt% of the total weight of the silicon content. Preferably, the silicon component may suitably comprise silicon compounds such as hydrates, carbides and oxides; pure silicon; and (3) silicon alloy.
As used herein, the term "carbonate" includes compounds having a group consisting of optionally substituted groups
Figure BDA0001889878250000021
A compound having a carbonate core structure. The fluorocarbonate may include one or more fluoro compounds, such as one, two, three, or four fluoro compounds. Preferred fluorocarbonates may include a fluoro or fluoroethylene carbonate (FEC)>
Figure BDA0001889878250000022
The anode can include a silicon-carbon composite, a first carbon component, and a conductor.
As used herein, the term "carbon component" includes materials that contain carbon as a major component, e.g., constitute an amount greater than about 95wt%, 96wt%, 97wt%, 98wt%, 99wt%, or 99.5wt% of the total weight of the carbon component. Non-limiting examples of carbon components may include amorphous carbon, graphite, diamond, fullerenes, carbon nanotubes, carbon nanofibers, hexagonal carbon, glassy carbon, carbon nanofoam, and carbenes.
As used herein, the term "conductor" includes materials having conductive properties for heat and/or electricity. Preferably, the conductor may have electrical conductivity, for example, through or via a material to transfer electrons.
Preferably, the first carbon component may suitably comprise graphite. The first carbon component may be graphite.
The silicon-carbon composite may include silicon, a second carbon component, and a coating. Preferably, the second carbon component may comprise graphite. The second carbon component may be graphite. Preferably, the coating may comprise carbon.
The electrolyte may include about 5wt% or more and less than about 10wt% of the fluorocarbonate based on the total weight of the electrolyte. For example, the electrolyte may include about 5wt% or more and less than about 10wt% FEC based on the total weight of the electrolyte.
The weight ratio of silicon to FEC can be obtained using the absolute amount of silicon in the unit cell of the lithium secondary battery and the absolute amount of FEC in the unit cell.
The absolute amount of FEC in the unit cell can be obtained by equation 1.
[ Eq.1 ]
Absolute amount of FEC in unit cell = injection amount of electrolyte x weight ratio of FEC to total composition of electrolyte
The absolute amount of silicon in the unit cell can be obtained by equation 2.
[ Eq.2 ]
Absolute amount of silicon = load level of anode per unit area x area of anode x content of active material x number of electrode stacks x content of silicon in active material
Preferably, the fluorocarbonate may form a film at the interface between the silicon component of the anode and the electrolyte.
In one aspect, a vehicle is provided that may include a lithium secondary battery as described herein.
Other aspects of the invention are disclosed below.
Drawings
These and/or other aspects of the invention will be apparent from and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:
fig. 1 illustrates an exemplary structure of an exemplary anode according to an exemplary embodiment of the present invention;
FIG. 2 is a cross-sectional view of an exemplary silicon-carbon composite according to an exemplary embodiment of the present invention;
fig. 3 shows a graph of exemplary cell performance depending on whether an electrolyte contains fluoroethylene carbonate according to an exemplary embodiment of the present invention;
fig. 4 shows a graph of performance of an exemplary unit cell according to concentration of fluoroethylene carbonate in an exemplary electrolyte according to an exemplary embodiment of the present invention; and
fig. 5 shows a graph showing the performance of a unit cell according to the weight ratio of silicon/fluoroethylene carbonate according to an exemplary embodiment of the present invention.
Detailed Description
In the following description, all elements of the embodiments of the present disclosure will not be described, and descriptions of what are known in the art or overlap each other in the embodiments will be omitted.
It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof, unless the context clearly dictates otherwise.
As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
Furthermore, unless specifically stated or apparent from the context, the term "about" as used herein should be understood to be within normal tolerances in the art, for example, within 2 standard deviations of the mean. "about" is understood to be within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05% or 0.01% of the specified value. Unless the context clearly indicates to the contrary, all numerical values provided herein are modified by the term "about".
In the following description, a vehicle refers to various devices for moving a transport object (such as a person, an object, an animal, etc.) from a departure point to a destination. Vehicles may include vehicles traveling on roads or tracks, vessels moving on the sea or river, and aircraft flying in the air using air.
In addition, a vehicle traveling on a road or a track may be moved in a predetermined direction by rotation of at least one wheel including a three-or four-wheeled vehicle, an engineering machine, a two-wheeled vehicle, a motor device, a bicycle, and a train traveling on a track.
Hereinafter, exemplary embodiments according to the present invention will be described in detail with reference to the accompanying drawings.
A lithium secondary battery generally includes a cathode, an anode, a separator, and an electrolyte. The cathode, anode and separator constituting the electrode structure of the lithium secondary battery according to the present invention may be implemented as those commonly used in the manufacture of conventional lithium secondary batteries.
The electrode includes an electrode active material. The electrode may be formed by applying an electrode slurry, in which an electrode active material, a solvent, and a conductive material are mixed, to a predetermined thickness on a current collector, and then drying and rolling the electrode slurry applied on the current collector.
The anode active material used for manufacturing the anode may be implemented using any anode active material as long as the anode active material can insert and extract (extract) lithium ions. The anode active material may include a material that may allow reversible adsorption or extraction of lithium and/or a metal material that may form an alloy with lithium.
Exemplary materials that may allow reversible adsorption or extraction of lithium may suitably include one or more materials selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesophase carbon microspheres, fullerenes, and amorphous carbon.
Examples of amorphous carbon may suitably include hard carbon, coke, or Mesophase Carbon Microspheres (MCMB) and mesophase pitch-based carbon fibers (MPCF) calcined at a temperature of about 1500 ℃ or less. The metallic material that may be alloyed with lithium may suitably comprise one or more metals selected from Al, si, sn, pb, zn, bi, in, mg, ga, cd, ni, ti, mn and Ge. These metal materials may be used alone, in combination, or in an alloy. In addition, such metals may be used in the form of composites mixed with carbon-based materials.
The anode active material may suitably comprise silicon. Preferably, the anode active material may suitably comprise a silicon-carbon composite. The anode active material including silicon has a high capacity but has a low lifetime characteristic due to excessive expansion upon charge and discharge. Accordingly, the present invention can provide a lithium secondary battery having significantly improved life characteristics by controlling the amount of silicon in an anode active material and the amount of fluoroethylene carbonate (FEC) in an electrolyte.
The anode active material according to the exemplary embodiment may suitably include a silicon component. The silicon component may suitably comprise one or more of silicon oxide, silicon particles and silicon alloy particles. For example, the silicon alloy may suitably include a solid solution including i) aluminum (Al), manganese (Mn), iron (Fe), titanium (Ti), and the like, and ii) silicon, a silicon-containing intermetallic compound, and a silicon-containing eutectic alloy, but the anode active material according to the present invention is not limited thereto.
The cathode active material according to an exemplary embodiment of the present invention may include a compound that allows reversible intercalation and deintercalation of lithium. Preferably, the cathode active material may suitably include i) one or more composite oxides including lithium, and ii) one or more metals selected from cobalt, manganese, and nickel.
The electrode according to the exemplary embodiment of the present invention may further include other components such as a dispersion medium as an additive, a conductive material, a viscosity modifier, and a filler.
The separator may prevent a short circuit between the cathode and the anode and provide a channel for lithium ions. The separator may suitably comprise i) a polyolefin-based polymer film, for example comprising one or more of polypropylene, polyethylene/polypropylene/polyethylene, polypropylene/polyethylene/polypropylene or multilayers thereof, ii) a microporous film, iii) a fabric or nonwoven fabric as known in the art. Preferably, a porous polyolefin film coated with a highly stable resin may be used for the separator.
The electrolyte may suitably include i) a lithium salt and ii) a nonaqueous organic solvent, and may further include an additive for improving charge/discharge characteristics, preventing overcharge, and the like. Preferably, the lithium salt may suitably comprise a metal selected from LiPF 6 、LiBF 4 、LiClO 4 、LiCl、LiBr、LiI、LiB 10 Cl 10 、LiCF 3 SO 3 、LiCF 3 CO 2 、LiAsF 6 、LiSbF 6 、LiAlCl 4 、CH 3 SO 3 Li、CF 3 SO 3 Li、LiN(SO 2 C 2 F 5 ) 2 、Li(CF 3 SO 2 ) 2 N、LiC 4 F 9 SO 3 、LiB(C 6 H 5 ) 4 、Li(SO 2 F) 2 N, liFSI (CF) 3 SO 2 ) 2 One or more lithium salts of NLi.
The nonaqueous organic solvent may suitably comprise one or more of a carbonate, an ester, an ether and a ketone. Examples of carbonates may suitably include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (EMC), ethylene carbonate, propylene Carbonate (PC), butylene Carbonate (BC), fluoroethylene carbonate (FEC), vinylene Carbonate (VC), and the like. The esters may suitably include gamma-butyrolactone (GBL), methyl acetate, ethyl acetate, n-propyl acetate, and the like. The ether may suitably comprise dibutyl ether or the like. The nonaqueous organic solvent may include a typical nonaqueous organic solvent in the related art, but is not limited thereto.
The non-aqueous organic solvent may also include aromatic hydrocarbon organic solvents. Examples of the aromatic hydrocarbon organic solvent may suitably include one or more of benzene, fluorobenzene, bromobenzene, chlorobenzene, cyclohexylbenzene, isopropylbenzene, n-butylbenzene, octylbenzene, toluene, xylene, mesitylene and the like.
Preferably, the electrolyte may include fluoroethylene carbonate.
Hereinafter, the lithium secondary battery according to various exemplary embodiments will be described in detail. In the following description, unless otherwise indicated, the units of measurement are weight% (wt%).
Fig. 1 illustrates an exemplary structure of an exemplary anode according to an exemplary embodiment of the present invention, and fig. 2 illustrates a cross-sectional view of an exemplary silicon-carbon composite according to an exemplary embodiment of the present invention.
The lithium secondary battery may include a cathode, an anode including silicon, a separator located or disposed between the cathode and the anode, and an electrolyte including fluoroethylene carbonate. Preferably, the weight ratio of silicon to FEC may be about 0.4 to about 0.8.
The weight ratio of silicon to FEC will be described further below.
As shown in fig. 1, the anode may include a silicon-carbon composite 1, a first carbon component (e.g., graphite) 2, and a conductor 3. For example, the silicon-carbon composite 1, graphite 2, and conductor 3 may be the active materials of the anode. For example, the silicon-carbon composite 1 may be manufactured by electrospinning a mixed solution containing a polymer material and silicon particles to prepare a one-dimensional structure composite, and by heat-treating the composite. Electrospinning can be performed at a humidity of about 36% or less and a temperature of about 34 ℃ or less. Electrospinning can be performed by applying a voltage of about 0.5kV/cm to 3.0kV/cm using an injection port of about 17 gauge to 25 gauge. The heat treatment may be performed by performing a primary heat treatment at a temperature of about 230-350 c for about 1 to 10 hours and then performing a secondary heat treatment at a temperature of about 500-900 c for about 1 to 7 hours. The secondary heat treatment may be performed under a mixed gas of an inert gas and a reducing gas.
The polymeric material may suitably comprise one or more selected from the group consisting of Polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylic acid and polyurethane.
The silicon-carbon composite 1 may suitably include silicon 10, a second carbon component (e.g., graphite) 11, and a coating 12. Preferably, amorphous silicon dioxide (SiO 2 ) A coating 12 is applied. For example, silicon 10 may be dispersed on the surface of graphite 11. The silicon-carbon composite according to an exemplary embodiment of the present invention may include a fiber mixture having a one-dimensional structure, which has excellent lithium ion conductivity and electrical conductivity by being composited with carbon as compared to silicon metal. In addition, the volume expansion during intercalation of lithium ions can be reduced by amorphous silica and graphite coated on silicon metal particles.
Fig. 3 illustrates performance of an exemplary unit cell depending on whether an electrolyte includes FEC according to an exemplary embodiment of the present invention.
As shown in fig. 3, when FEC is not included in the electrolyte, the capacity retention rate may drastically decrease. FEC may form a flexible film between the silicon and the electrolyte to inhibit reactions that may occur between the silicon and the electrolyte. The film may represent a passivation film, referred to as a Solid Electrolyte Interface (SEI).
Fig. 4 shows a graph showing the performance of an exemplary unit cell according to the concentration of fluoroethylene carbonate in an electrolyte according to an exemplary embodiment of the present invention.
As shown in fig. 4, when FEC is present in an amount of about 10wt% based on the total weight of the electrolyte, charge-discharge efficiency may be reduced.
Preferably, the FEC may be present in an amount of about 5wt% or more and less than about 10wt% based on the total weight of the electrolyte. When the content of FEC is more than about 10wt%, charge/discharge efficiency may decrease due to an increase in thickness of the film, and thus the life of the lithium secondary battery may decrease. When the content of FEC is equal to or less than about 5wt%, the film may not be completely formed, and thus, a secondary (suberate) reaction between silicon and electrolyte may be increased by cracking the film or not forming a portion of the film due to volume expansion of silicon, and thus, the service life of the lithium secondary battery may be reduced. Therefore, it is necessary to determine an optimal ratio between the absolute amount of silicon and the absolute amount of FEC.
Fig. 5 shows a graph showing cell performance according to a silicon/FEC weight ratio according to an exemplary embodiment of the present invention.
The weight ratio of silicon to FEC can be obtained using the absolute amount of FEC in the unit cell and the absolute amount of silicon in the unit cell. According to an exemplary embodiment of the present invention, when the weight ratio of silicon to FEC is about 0.4 to 0.8, the capacity retention rate may be significantly increased.
The absolute amount of FEC in the unit cell can be obtained by equation 1.
[ Eq.1 ]
Absolute amount of FEC in unit cell = injection amount of electrolyte x weight ratio of FEC to the entire composition of electrolyte.
For example, the absolute amount of silicon in the unit cell can be obtained by equation 2.
[ Eq.2 ]
Absolute amount of silicon in unit cell = load level of anode per unit area x area of anode x content of active material x number of electrode stacks x content of silicon in active material.
The weight ratio of silicon to FEC can be obtained as the absolute amount of silicon in the unit cell/the absolute amount of FEC in the unit cell. The capacity retention ratio shown in the y-axis of fig. 5 can be obtained using the discharge capacity (Ah) in the implementation cycle/the discharge capacity (Ah) in the first cycle.
As described above, the optimum ratio of the absolute amount of silicon to the absolute amount of FEC may suitably be about 0.4 to 0.8. When the absolute amount of silicon is changed, the deterioration of the cell performance can be prevented by appropriately controlling the absolute amount of FEC to form a passivation film on silicon according to the change of the absolute amount of silicon.
According to various exemplary embodiments of the present invention, a lithium ion battery may be manufactured by effectively forming a passivation film on silicon without degrading the performance of the unit cell.
Although exemplary embodiments of the present invention have been described with reference to the accompanying drawings and tables, those skilled in the art will understand that the inventive concepts may be embodied in different forms without departing from the scope and spirit of the disclosure, and should not be construed as limited to the embodiments set forth herein. The disclosed embodiments have been described for illustrative purposes and not for limitation.

Claims (7)

1. A lithium secondary battery, comprising:
a cathode;
an anode comprising a silicon component;
a separator disposed between the cathode and the anode; and
an electrolyte comprising fluoroethylene carbonate FEC,
wherein the electrolyte comprises FEC in an amount of 5wt% or more and less than 10wt% of the total weight of the electrolyte,
wherein the weight ratio of the silicon to the FEC is 0.4-0.8,
wherein the weight ratio of the silicon to the FEC is obtained using the absolute amount of the silicon in the unit cell of the lithium secondary battery and the absolute amount of the FEC in the unit cell,
wherein the absolute amount of the FEC in the unit cell is obtained by the following equation 1:
[ Eq.1 ]
Absolute amount of fec=injection amount of electrolyte×weight ratio of FEC to total weight of electrolyte, and
wherein the absolute amount of the silicon in the unit cell is obtained by the following equation 2:
[ Eq.2 ]
Absolute amount of silicon = load level of the anode per unit area x area of the anode x content of active material x number of electrode stacks x content of silicon in the active material.
2. The lithium secondary battery of claim 1, wherein the anode comprises a silicon-carbon composite, a first carbon component, and a conductor.
3. The lithium secondary battery of claim 2, wherein the silicon-carbon composite comprises the silicon component, a second carbon component, and a coating.
4. The lithium secondary battery of claim 2, wherein the first carbon component comprises graphite.
5. The lithium secondary battery of claim 3, wherein the second carbon component comprises graphite.
6. The lithium secondary battery according to claim 1, wherein FEC forms a film at an interface between the silicon component of the anode and the electrolyte.
7. A vehicle comprising the lithium secondary battery according to claim 1.
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